This invention relates to semiconductor devices and to a method for attaching semiconductor devices to an associated package. More particularly, the invention relates to a metallization system having improved mechanical and thermal properties whereby the semiconductor device may be metallurgically bonded to a metal package member. The invention may be most advantageously applied to power transistors, but is equally applicable to other devices.
A transistor or other semiconductor device, in die form, is metallurgically bonded to a metal member in a metal or metal and plastic package. The package then functions as a mechanical support for the device, provides electrical contact to one terminal of the device, and serves to dissipate heat generated within the device. Accordingly, the bonding material must be low in both terminal and electrical resistance. The material must also be able to survive repeated stressing and maintain its integrity. This is occasioned by the fact that as the device is repeatedly turned on and off the die temperature as well as the package temperature rises and falls. Because the die and the package can have different coefficients of thermal expansion, they will expand at different rates and in different proportions as the operating temperature changes. The composite structure consisting of die, die bond, and package will, thus, be stressed as the temperature changes.
Two different techniques are in common use for metallurgical die bonding. These can be roughly classified as hard solder bonding based on gold alloys and soft solder bonding based on lead or tin alloys. The hard solders form strong bonds having excellent thermal and electrical properties, but the high strength of the hard solder can cause the semiconductor die to fracture or the die and metal member to deform elastically. Hence, for dice larger than approximately 140 mils on a side the die will fracture during temperature cycling because the hard solder transfers the stress to the die without plastic deformation in the die bond. Lead based soft solders are plastic enough to accommodate the thermal expansion mismatch between the die and the package, but soft solder bonds are susceptible to metal fatigue after repeated temperature cycles. Device reliability can thus be degraded. Thicker layers of solder can reduce the stress in the solder and lessen the fatigue problem, but only at the expense of undesirably higher thermal resistance.